1. Field of the Invention
This invention relates to ionomer compositions with improved high temperature utility compared with standard ionomers. The compositions are blends of two substantially different ethylene/carboxylic acid copolymers or derived ionomers. The blend components are, or derive from, ethylene/carboxylic acid copolymers prepped under markedly different polymerization conditions, and also, preferably, different acid levels.
2. Description of Related Art
Copolymers of ethylene and an unsaturated carboxylic acid such as (meth)acrylic acid, optionally with another comonomer, and their derived ionomers are well known. These copolymers typically contain at least 50 weight percent and up m about 95 weight percent ethylene. Not unexpectedly, they have some characteristics which reflect crystallinity somewhat similar to that of polyethylene. The polar acid groups in such acid copolymers lead differences from, and certain advantages, as well as some disadvantages compared with polyethylene itself. When the acid copolymers are neutralized, the resulting ionomers contain ionic bonds which lead to an additional difference, and some advantages over the acid copolymer itself. Ionomers contain effective crosslinking at use temperatures, yet thermoplastic processibility at melt temperatures. Ionomer properties thus display characteristics which reflect a crosslinked nature, and an ionic nature. Ionomers have higher tensile strength, greater clarity, better abrasion resistance and higher stiffness than acid copolymers with comparable melt index (MI) and comonomer level.
The higher the acid level, the higher the degree of ionic character possible, since there are more acid groups to be neutralized with metal cations. Neutralization increases molecular weight (particularly weight-average rather than the underlying number-average chain length) and viscosity. MI decreases on neutralization. Thus the acid copolymers used to make ionomers are polymerized to a much lower molecular weight (higher MI) than typical for acid copolymers (other than those for adhesive use where high MI is the norm), and then neutralized to higher molecular weight (lower MI) via ionic crosslinking. The molecular weight required to achieve good mechanical properties in ionomers is thus achieved, in part, by crosslinking rather than by increasing degree of polymerization per se as is necessary with (un-neutralized) acid copolymers for uses other than for adhesives.
Typically, ionomers are produced from acid copolymers having an MI of 20 to 80 g/10 min. The MI of the neutralized ionomers, for good mechanical properties is typically less than about 3.0 g/10 min. and often less than 1.0 g/10 min. and even as low as 0.1 g/10 min. Acid copolymers themselves with good mechanical properties would also have an MI of 3.0 or even lower. However, an MI below 3.0 g/10 min. for either acid copolymers or ionomers corresponds to a viscosity which can lead to poor processibility.
The interspersed copolymerized acid units however, modify, and may reduce the level of crystallinity compared with polyethylene and, unfortunately, reduce the melting point and upper use temperature to well below that of polyethylene itself. Neutralization generally further reduces the freezing point somewhat and also may reduce the amount of crystallinity. Increasing the use temperature of ionomeric copolymers, while maintaining their essential ionomer character, has become a holy grail. Typical commercial ionomers, such as those sold under the trade name Surlyn.RTM. by E. I. du Pont de Nemours and Company, derive from acid copolymers with about 9 to 20 weight percent (meth)acrylic acid comonomer. As normally prepared, both the acid copolymers and their derived ionomers have differential scanning calorimetry (DSC) melting points which are in the region of about 81.degree. to about 96.degree. C. This is considerably below that of low density (branched) polyethylene prepared under generally comparable conditions, which has a melting point of about 115.degree. C. For many uses it would be desirable to have an ionomer with a melting point above 100.degree. C., and as high as 110.degree. C. or even higher.
U.S. Pat. No. 4,248,990 (Pieski), discloses that the polymerization pressure and temperature both have a strong effect on the stiffness of acid copolymers. Pieski considered polymerization at low pressure using `normal` temperatures, and at low temperature using `normal` pressures equivalent options to producing the polymers of his invention. When low polymerization temperature alone, i.e., at `normal` pressures was used the Vicat Softening temperature, stiffness, and tensile yield strength increased dramatically for acid copolymers with about 9 to 15 weight percent methacrylic acid, when polymerization temperature was decreased from 250.degree. to 160.degree. C. The increased softening temperature corresponds to an increase in the melting points. This increased temperature was attributed to a change in the randomness of the acid and ethylene groups along the polymer chain. At the same acid level, an increase in the number of acid diads and triads occurs. This results in less break up of the polyethyene sequences in the polymer for a given acid level, and a higher melting point, nearer that of polyethylene. Pieski discloses, and his data show, that as an alternative to low temperature, low pressure also produces more diads and triads. The two different polymerization conditions were considered alterative modes of producing polymer of his invention.
However, low temperature and low pressure may not at all be equivalent alternatives. Based on analogy with polyethylene polymerization, at lower polymerization temperatures, less short chain branching occurs, and this also contributes to higher crystallinity and higher melting point. By contrast, polymerization at low pressure at normal temperatures produces higher levels of short chain branching and hence lower crystallinity--just the opposite of what is required for high temperature behavior. Interestingly, Pieski's data show only slightly higher stiffness for low pressure polymerization, and softening temperature data are entirely absent. In contrast to Pieski's theories of the all importance of sequence distribution, as a result of the present invention, it is now believed that low branching is at least equally, and probably more important. As a result, low pressure polymerization is specifically excluded in the present invention.
There is, however, a significant decrease in polymer productivity when employing low temperature polymerization. Heat evolved from the polymerization, which will be proportional to the polymerization rate, will determine polymerization temperature for a given monomer feed temperature, when polymerization is run, as it typically is, under largely adiabatic conditions. The temperature difference between feed and polymerization temperature will thus be a measure of polymerization rate. Thus, very generally, for a 40.degree. C. feed, productivity can be reduced by a factor of 120/210, or by about 43 percent when the polymerization temperature is reduced from 250.degree. to 160.degree. C.
A further problem with low temperature polymerization of acid copolymers is that phase separation of monomer and polymer can occur. Normal polymerization conditions of high pressure and high temperature allow polymerization in one phase. Phase separation is also more acute at higher acid levels, even at normal polymerization temperatures, but particularly at low polymerization temperatures. When phase separation occurs, non-uniform polymerization results.
The melting point of ethylene/carboxylic acid copolymers, polymerized at any given temperature, becomes closer to that of polyethylene as the molar amount of acid comonomer copolymerized into the polymer is reduced. For instance, the melting point of acid copolymers polymerized at 250.degree. C. increases about 7.degree. C., from about 94.degree. to 101.degree. C., when the acid content is decreased from about 4 to 2 mole percent, and from about 104.degree. to 111.degree. C. at 180.degree. C. polymerization temperature. (On a weight basis this would correspond to about 13 to 6 weight percent methacrylic acid and about 11 to 5 weight percent acrylic acid). The highest melting point achievable in acid copolymers can be obtained therefore, by a combination of low temperature polymerization and low acid levels. For the derived ionomers however, low acid levels allow development of less ionomer character when neutralized since, with a lower amount of acid to be neutralized, fewer ions can be incorporated.
With regard to improved processibility, recently it has been discovered that ionomers with almost equivalent properties to standard ionomers of MI below about 3.0 g/10 min., but with much better flow, can be obtained when the MI of the parent acid copolymer is as high as 300 g/10 min., rather than the normal 20 to 80 g/10 min., provided they are neutralized to a relatively high degree. Such polymers are neutralized to at least 40%, compared with less than 30% for some standard ionomers. This large MI reduction necessitates parent acid copolymers having relatively high acid levels; about 10 weight percent for methacrylic acid, and preferably above. The final MI of such materials can be much higher than regular ionomers (e.g., as high as 7.0 g/10 min). The higher starting MI allows a higher percent neutralization of acid groups present before a given MI is obtained. In addition, high acid levels allow a higher level of ion incorporation for a given level of neutralization. Naturally, these new ionomers have excellent melt processibility to the extent that processibility is affected by good melt flow. However, melt processibility is not just a function of melt flow, but also of crystallization characteristics.
The concept of blending a low melting point resin with a high melting point resin is well known. Blends of standard ionomers, with their low melting point, with polyethylene with a much higher melting point are, however, somewhat incompatible, and as a result have certain poorer properties including lower melt strength and loss of clarity. While commercial compositions which are blends of ionomer and a major portion of polyethylene (high density) do exist, their properties are substantially different from those being sought here, which are essentially those of a pure ionomer.
Blending different ionomers or ionomers with acid copolymers is also well known, and for typical copolymers which have acid levels of 9 weight percent and above, incompatibility is not a problem. Ions are believed to be significantly labile, so that acid copolymers blended with ionomers produce compositions where the ions are associated with all acid groups present.
Such blending has taken on particular importance in certain end uses such as golf ball materials. Thus, U.S. Pat. No. 5,397,840 (Sullivan et al.) discloses blends of ionomers and acid copolymers for golf ball cover materials. Many similar patents disclose ionomer blends. However, in all these cases, there is no disclosure of blends where the acid copolymers, from which the ionomer components are derived, are prepared under vastly different polymerization conditions, and where at least one blend component has a parent acid copolymer with high MI.
There is a need for ionomers which have good mechanical properties, particularly at higher temperatures, in order to increase end-use temperature, yet which still have excellent or even improved processabilty.